1. Field of the Invention
This invention relates to a thermoplastic elastomer, more particularly a thermoplastic elastomer having a high strength and an excellent fluidity, flexibility, oil resistance, and rubber elasticity.
Thermoplastic elastomers now can be processed by the same methods as used for processing thermoplastic resins, for example, injection molding, blow molding, rotational molding, and extrusion molding, and various compositions having a required rubber-like flexibility are commercially available and have been utilized for various purposes which require a good processability and easy recycle use, compared with crosslinked rubbers of the prior art.
The thermoplastic elastomer has a molecular design, containing in a polymer system soft segments exhibiting rubbery properties at the use temperature thereof and hard segments such as crystal, glass, etc., which can be regarded as pseudo-crosslinks, behaving in the same way as a crosslinked rubber at the use temperature thereof and exhibiting a similar behavior to general thermoplastic resins at the processing temperature.
Among various thermoplastic elastomers, polyolefin elastomers are primarily used in the fields of automobiles and cables due to their superior weatherability and heat resistance.
2. Description of the Related Art
An olefinic thermoplastic elastomeric composition comprising a blend of a partially crosslinked monoolefin copolymer rubber and a polyolefin resin is disclosed in Japanese Patent Publication (Kokoku) No. 53-34210, among others. This composition, although having an excellent flexibility and fluidity, has a low strength and poor rubber elasticity, compared with vulcanized rubber. As an elastomer composition in which these drawbacks have been eliminated, there is known an olefinic thermoplastic elastomer composition comprising a blend of a completely crosslinked ethylene-propylene-nonconjugated diene rubber (EPDM) and a polyolefin resin, as disclosed in Japanese Patent Publication (Kokoku) No. 55-18448, among others. This composition, however, has an inferior fluidity, although having performances comparable with those of vulcanized rubber, and there is still a need for an improvement thereof.
The major cause of a lowering of other characteristics when one drawback is eliminated, is considered due to the fact that the rubber component used is an amorphous and random copolymer, and further, is an EPDM with a narrow molecular weight distribution containing unsaturated groups. Such a copolymer rubber, although flexible, has a very low strength, and crosslinking is required for an enhancement of this strength.
Nevertheless, although the heat resistance and compression permanent set, etc., can be improved by a partial crosslinking with, for example, an organic peroxide, there is relative little improvement in the tensile strength, and accordingly, as the amount of rubber component is increased, a complete crosslinking is required to maintain the strength, but this markedly lowers the fluidity.
As a means for avoiding a lowering of the fluidity, a mineral oil type softening agent has been added in a great amount, but this results in undesirable effects such as an increased kneading time when adding the oil, a lower strength, and bleeding, etc.
A crosslinked resin composition using a crystalline ethylene-.alpha.-olefin is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-152753, but since the molecular weight of the crystalline ethylene-.alpha.-olefin used is as relatively low as 0.01 to 100 g/10 min. in terms of the melt index at 190.degree. C., the green strength as a soft rubber component is low, and further the crosslinking characteristic is poor for the same reason. Consequently, this has proved to be unsatisfactory when trying to obtain a composition with a well-balanced flexibility, strength, and rubber elasticity.